Spectrally sensitizing dyes are used in silver halide photographic materials for the purpose of photosensitizing the material to light of a specific wavelength. Furthermore, dyes to provide shielding from light of specified wavelengths are also used. Many dyes of various structure which absorb visible light and which can be used for these purposes are known. However, in recent years semiconductor lasers which generate near infrared light are used as light sources and it has also been necessary to use dyes which absorb near infrared light in silver halide photographic materials.
Dyes which absorb near infrared light can be obtained by extending the conjugation system of dyes which absorb visible light. In particular, adsorption in the near infrared can be achieved by extending the length of the conjugated methine chain in cyanine dyes which can be used as photographically useful sensitizing dyes.
However, the weight which is added to the photographic material increases as a result of the increase in molecular weight of the dyes when the structure is modified in this way. This brings about undesirable effects such as that the formulation for the manufacture of the photographic materials is limited or that the weight of unwanted material, such as the dyes and their degradation products which are washed out from the photosensitive material during development and processing, is increased and this increases the burden on the effluent depolluting treatment systems.
On the other hand, it is known that the stability of a cyanine dye falls as the conjugated methine chain is extended.
Hence, dyes with which the absorption of light having a long wavelength, and especially light in the near infrared region, can be achieved with as short a conjugated methine chain length as possible are desired. However, to be useful as spectrally sensitizing dyes for photographic purposes, these dyes must exhibit an adequate spectral sensitivity in a photographic material, but this is not always the case in practice.